1. Field of the Invention
The present invention relates to a method for eliminating flickers of a liquid crystal display (LCD), and more particularly to a method for the elimination of flickers by adjusting the areas of storage capacitors.
2. Description of the Related Art
Three-level capacitive coupling driving is a driving method often applied to a liquid crystal display for the time being. According to the driving method, there are three different levels for the scanning signals 17 working on the scanning lines 11, namely Vgh, Vgl and Vgc, as shown in FIG. 1, wherein the thin film transistor 13 of the pixel 10 is turned on at level Vgh and turned off at level Vgl. If the thin film transistor 13 is on, the data signals working on the data lines 16 are written into the liquid-crystal capacitor 14. The gate pulse compensation level Vgc works on the scanning lines 12 to enable the storage capacitor 15 to generate a coupled voltage. The coupled voltage is intended to compensate for the voltage drop in the voltage level of the liquid crystal capacitor 14 with a view to eliminating flickers that may otherwise result from the voltage drop.
In order to measure the flicker levels of the liquid crystal display 20, positions of a plurality of pixels P1-P49 arranged in a grid-point matrix are shown on the panel, as shown in FIG. 2. Then the flicker levels appearing on the pixels at each grid-point are measured with an optical measuring instrument. The positions of the pixels are defined by the readings shown on the x-axis and y-axis in the figure.
FIG. 3 is a diagram of the distribution of the measurements related to the flicker levels of the pixels P1-P49 in FIG. 2. As regards the liquid crystal display 20, the flickers occurring in the pixels at the center and the right portion separately measure approximately −30 dB or less in the flicker level, but those at the left portion (as indicated by the marks found in the circle) mostly measure more than −30 dB in the flicker level. Since flickers that measure approximately −30 dB or less in the flicker level are invisible to the naked eye, it is believed that no flicker exists in the pixels either at the center portion or at the right portion but most of the pixels at the left portion have flickers that are perceptible.
In general, one may adjust the level Vgc for a liquid crystal display by turning the knob of a variable resistor, and solve the flicker problem of the screen by varying the level Vgc. Unfortunately, it is impossible to solve the flicker problem of the screen by means of one single level Vgc, as verified with a test conducted on the three curves 41, 42 and 43 shown in FIG. 4. The three curves are plotted using the measurements about the flicker levels appearing on selected pixels on the left, center, and right portion of the liquid crystal display respectively, where the pixels selected for such a purpose are limited to a certain area of each of the three above-mentioned portions. By changing the level Vgc of the transverse axis, one draws the curves to describe the variable flicker level of the pixels in the three portions. Since the transverse axis readings (−10.10V) that correspond to minimum readings shown on the respective lengthwise axis of the curves 42 and 43 are roughly equal in magnitude, flickers appears on pixels at the center as well as those on the right portion can be eliminated at the same level Vgc that is, −10.10V. As shown in FIG. 4, at a level Vgc equal to −10.10V, the flicker level described by the curve 41 is greater than −30 dB, thus it is impossible to eliminate flickers appearing on the pixels at the left portion using the same voltage level. Since the transverse axis reading that corresponds to the minimum reading shown on the lengthwise axis of the curve 41 is −10.30V, the difference between it and the above-mentioned transverse axis reading that corresponds to minimum readings shown on the respective lengthwise axes of the curves 42 and 43 is 0.2V approximately.
In short, conventional technology fails to address the flicker problem of the screen of a liquid crystal display using one single adjustable level Vgc, thus there is great demand for a method intended to solve the flicker problem of a liquid crystal display.